The present invention relates generally to a process and apparatus for the fabrication of a fiber reinforced polymer composite structure and, more particularly, to such a process and apparatus for use in fabricating fiber reinforced composite structures using a vacuum bag molding process.
Fiber reinforced polymer composite structures have been manufactured for a number of years and may take many shapes and sizes. The general process for manufacturing these composite structures involves the incorporation of one or more layers of a fibrous reinforcement within a resinous material. This is most commonly achieved through the employment of a dry lay up technique. The dry lay up technique involves applying to a mold one or more layers of a dry fiber reinforcing material and then either spraying, brushing or otherwise applying resin in order to thoroughly impregnate, or "wet", the material with the resin. Once the layers of fiber have been thoroughly wetted, the resin cures or solidifies to form the resulting fiber reinforced polymer composite structure.
Historically, dry lay up has been a labor intensive process because the resin is applied by hand to ensure that all areas of the fibrous reinforcement are thoroughly wetted. Typically, this is done one layer at a time, the fibrous material being placed on the mold and then the resin being applied by hand until several layers are on the mold and ready to cure. In addition to the long process time and high production costs associated with this process, it also has the disadvantage of releasing high quantities of volatile organic compounds, or VOCs, which are emitted from the resin into the air. As regulations on VOC emissions become more and more stringent, the severity of this problem continues to increase.
More recently, vacuum bag molding processes have been developed to overcome some of the disadvantages associated with the dry lay up technique. Vacuum bag molding involves the use of a resin impervious, flexible vacuum bag, liner or sheet marginally sealed over the mold to enclose dry fiber reinforcing material, or "lay up," placed on the mold. A vacuum is applied to the interior of the enclosure to pull the vacuum bag tight against the fiber lay up. A resin is then introduced into the enclosure through a resin inlet in the vacuum bag to wet the dry fiber lay up. Application of the vacuum causes the fibrous materials to pack together and take the shape of the mold. The vacuum also draws the resin into and through the fiber lay up to saturate the lay up. The mold is kept under vacuum while the resin cures or solidifies to form a fiber reinforced composite structure having the shape of the surface of the mold.
Although the vacuum bag molding process effectively reduces the amount of labor required, as well as the quantity of VOCs emitted by trapping them within the sealed enclosure, it also has a number of disadvantages. For example, the tight packing of the layers of dry fiber caused by the vacuum bag being collapsed against the fiber makes it difficult for the resin to diffuse through the fiber material and to flow to the outer margins of the mold. This can result in non-uniform distribution of the resin throughout the fibrous lay up, compromising the structural integrity of the composite structure. To reduce the risk of non-uniform distribution, an operator must assist the flow of resin through the fiber layers using a squeegee, roller, or some other suitable device on the outside of the vacuum bag to force the resin into areas which might not otherwise be reached, increasing the amount of labor, time and expense associated with the molding process.
Use of a distribution medium has been suggested to facilitate the uniform distribution of resin over the fiber material in a vacuum bag molding process, thus ensuring thorough wetting of the fiber. For example, U.S. Pat. No. 4,902,215 discloses an apparatus for achieving the uniform distribution of resin whereby a woven mesh made of non-resin absorptive monofilaments is placed on top of the fiber material between it and the flexible bag. This mesh acts to separate the upper surface of the fabric and the lower face of the bag, thus providing passageways through which the resin may freely and uniformly flow through the mold without the need for operator assistance. After the resin cures, the mesh is removed with the aid of a resin permeable peel ply film which has been inserted between the mesh and the fiber layers.
U.S. Pat. No. 5,316,462 discloses an alternative apparatus for uniformly distributing the resin in which channels are present on the side of the flexible vacuum bag facing the fiber material. Upon application of a vacuum and the subsequent collapse of the bag onto the fiber layers, these channels provide passageways through which the resin may freely and uniformly flow through the mold without the need for operator assistance. After the resin cures, the bag is lifted from the mold and away from the composite structure and then cleaned in order to remove solid resin present in the channels of the bag.
While the above approaches attempt to address the problems associated with the need for uniform distribution of resin, both result in the generation of additional solid waste since the resin distribution mesh and vacuum bag must be discarded. Alternatively, if the resin distribution mesh or vacuum bag with channels are to be reused, the mesh and the channels of the vacuum bag must be thoroughly cleaned before they can be used again, resulting in time consuming process delays. In addition, neither of the proposed distribution mediums impart additional strength to the resulting composite structure beyond that accorded by the fiber layers.
U.S. Pat. No. 2,913,036 (Smith) discloses another apparatus for promoting a uniform distribution of resin in which rigid conduits, constructed of metal or a cured fiberglass composite structure, are placed between layers of the dry lay up material. Holes are drilled into the conduits such that resin flows along the conduits and is distributed to the dry lay up material by flowing through the holes in the conduits. Smith also discloses that if the conduits are not to be included in the final composite structure, the conduits may be laid on top of the dry lay up material and broken away from the structure after the resin has cured.
However, the rigid conduits, constructed of metal or cured fiberglass composite structure, do not bond adequately with the dry lay up material upon curing of the resin. Therefore, as described by Smith, to incorporate the conduits into the composite structure for strengthening the structure the conduits must be placed between the layers of dry lay up material. This substantially increases the weight of the composite structure because more layers of dry lay up material are required. For example, in Smith, two layers of material are used beneath the conduits and another two layers overlay the conduits. In comparison, composite structures generally use only two or three layers of lay up material. Use of Smith's apparatus may also result in poor surface quality and part dimension consistency. As the rigid conduits generate a space between the fiber layers, resin rich regions will form in the space, resulting in a non-uniform curing and surface sink, shrinkage and warpage.